The human retrovirus, human immunodeficiency virus (HIV) causes Acquired Immunodeficiency Syndrome (AIDS), an incurable disease in which the body's immune system breaks down leaving the victim vulnerable to opportunistic infections, e.g., pneumonia, and certain cancers, e.g., Kaposi's Sarcoma. AIDS is major global health problem. The Joint United Nations Programme on HIV/AIDS (UNAIDS) estimates that there are now over 34 million people living with HIV or AIDS worldwide, some 28.1 million of those infected individuals reside in impoverished sub-Saharan Africa. In the United States, one out of every 250 people is infected with HIV or have AIDS. Since the beginning of the epidemic, AIDS has killed nearly 19 million people worldwide, including some 425,000 Americans. AIDS has replaced malaria and tuberculosis as the world's deadliest infectious disease among adults and is the fourth leading cause of death worldwide.
There is still no cure for AIDS. There is, however, an armamentarium of antiretroviral drugs that prevent HIV from reproducing and ravaging the body's immune system. One such class of drugs are the reverse transcriptase inhibitors, e.g., abacavir, delaviridine, didanosine, efavirenz, lamivudine, nevirapine, stavudine, zalcitabine, and zidovudine, which attack an HIV enzyme called reverse transcriptase. Another class of drugs is the protease inhibitors, e.g., amprenavir, indinavir, nelfinavir, ritonavir, and saquinavir, which inhibit HIV enzyme protease.
First introduced in 1995, these protease inhibitors are widely used for the treatment of HIV infection alone or in combination with other antiretroviral drugs. Today, approximately 215,000 of the estimated 350,000 patients receiving treatment for HIV infection in the United States take at least one protease inhibitor.
Highly active antiretroviral drug therapy (HAART) is a widely used anti-HIV therapy that entails triple-drug protease inhibitor-containing regimens that can completely suppress viral replication (Stephenson, JAMA, 277: 614-6 (1997)). The persistence of latent HIV in the body, however, has been underestimated. It is now recognized that there exists a reservoir of HIV in perhaps tens of thousands to a million long-lived resting “memory” T lymphocytes (CD4), in which the HIV genome is integrated into the cells own DNA (Stephenson, JAMA, 279: 641-2 (1998)). This pool of latently infected cells is likely established during primary infection.
Such combination therapy is often only partially effective, and it is unknown how much viral suppression is required to achieve durable virologic, immunologic, and clinical benefit (Deeks, JAMA, 286: 224-6 (2001)). Anti-HIV drugs are highly toxic and can cause serious side effects, including heart damage, kidney failure, and osteoporosis. Long-term use of protease inhibitors has been linked to peripheral wasting accompanied by abnormal deposits of body fat. Other manifestations of metabolic disruptions associated with protease inhibitors include increased levels of triglycerides and cholesterol, pancreatitis, atherosclerosis, and insulin resistance (Can et al., LANCET, 351: 1881-3 (1998)). The efficacy of current anti-HIV therapy is further limited by the complexity of regimens, pill burden, and drug-drug interactions. Compliance with the toxic effects of antiretroviral drugs make a lifetime of combination therapy a difficult prospect and many patients cannot tolerate long-term treatment with HAART. There is an urgent need for other antiviral therapies due to poor adherence to combination therapy regimes, which has led to the emergence of drug-resistant strains of HIV. Other drugs may improve compliance by substantially reducing the daily “pill burden” and simplifying the complicated dietary guidelines associated with the use of current protease inhibitors.
The HIV virus enters the body of an infected individual and lives and replicates primarily in the white blood cells. The hallmark of HIV infection, therefore, is a decrease in cells called T-helper or CD4 cells of the immune system. The molecular mechanism of HIV entry into cells involves specific interactions between the viral envelope glycoproteins (env) and two target cell proteins, CD4 and a chemokine receptor. HIV cell tropism is determined by the specificity of the env for a particular chemokine receptor (Steinberger et al., PROs. NATL. ACAD. ScI. USA. 97: 805-10 (2000)). T-cell-line-tropic (T-tropic) viruses (X4 viruses) require the chemokine receptor CXR4 for entry. Macrophage (M)-tropic viruses (R5 viruses) use CCR5 for entry (Berger et al., NATURE, 391: 240 (1998)). T-tropism is linked to various aspects of AIDS, including AIDS dementia, and may be important in disseminating the virus throughout the body and serving as a reservoir of virus in the body.
CD8+ T-cells secrete soluble factor(s) capable of inhibiting both R5- and X4-tropic strains of HIV and that are believed to play a critical role in vivo in antiviral host defense (Garizino-Demo et al., PROC. NATL. ACAD. SCI. USA, 96:111986-91 (1999)). These inhibitory factors include CC-chemokines (Cocchi et al., SCIENCE, 270: 1811-5 (1995); Horuk et al., J. BIOL. CHEM., 273: 386-91 (1998); Pal et al, SCIENCE, 278: 695-8 (1997)), that bind to the CCR5 coreceptor and inhibit R5 viral entry into cells (Garizino-Demo et al., PROC. NATL. ACAD. SCI. USA., 96: 111986-91 (1999); Liu et al., CELL, 86: 367-77 (1996); Samson et al., NATURE, 382:722-5 (1996); Scarlatti et al., NAT. MED., 3: 1259-65 (1997)) as well as less well characterized soluble factor(s) produced by CD8+ T-cells and termed CD8+ T-cell antiviral factor(s) (hereinafter, CAF) capable of inhibiting both R5 and X4 HIV (Walker et al., SCIENCE, 234: 1563-6 (1986); Chen et al., AIDS REs. Hum RETROVIRUSES, 9:1079-86 (1993); Mackewicz et al., PROC. NATL. ACAD. SCI. USA, 92:2308-12 (1995); Mackewicz et al., J. GEN. VIROL., 81 Pt.S: 1261-4 (2000); Leith et al., AIDS, 11: 575-80 (1997); Le Borgne et al., J. VIROL., 74: 4456-64 (2000); Tomaras et al., PROC. NATL. ACAD. SCI. USA, 97: 3503-8 (2000)). These CC-chemokines, however, do not account for all CAF antiviral activity released from these cells, particularly since CAF can inhibit the replication of X4 HIV strains that use CXCR4 and not CCR5 as a coreceptor. The identity of the factor(s) released from CD8+ T-cells capable of inhibiting X4 HIV has remained elusive.